Steven J. Britz

AMELIORATION OF UV‐B DAMAGE UNDER HIGH IRRADIANCE. I: ROLE OF PHOTOSYNTHESIS

Sensitivity to ultraviolet‐B radiation (UV‐B,280–315 nm) is generally reduced when background irradiance is high. We tested the involvement of photosynthesis in the amelioration of UV‐B damage by treating plants at high PAR (photosynthetically‐active radiation, 400–700 nm; 1000 μmol m‐2 s‐1) with supplemental UV‐B at double ambient levels of biologically‐effective radiation (18 kJ m‐2d‐1) and either “ambient” (450 μmol mol‐1) or short term elevated (750 μmol mol‐1) CO2 levels. Responses to UV‐B were assessed by photosynthetic gas exchange, leaf expansion and production of UV‐absorbing compounds (presumptive flavonoids) in cultivars of cucumber (Cucumis sativus L.) previously demonstrated to be relatively sensitive (cv. Poinsett) and insensitive (cv. Ashley) to UV‐B. Except for marginal leaf interveinal chlorosis observed in Poinsett, both cultivars responded similarly. UV‐B had little direct effect on leaf photosynthesis, but it did cause reductions in leaf area and corresponding increases in leaf dry matter per area. Increased CO, stimulated plant growth, counteracting the effect of UV‐B on leaf growth and indicating an important role for photosynthesis. In contrast, the accumulation of UV‐absorbing flavonoid compounds was enhanced by UV‐B exposure but was not affected by COz enrichment.

Spectral quality of two fluorescent UV sources during long-term use

The characteristics of a fluorescent ultraviolet (UV) lamp (UVB‐313), UV‐B transmitting cellulose diacetate (CA) and UV‐B absorbing polyester (PE) films were determined during actual use. Although lamp emission was stable between 70 and 386 h of burn time (longer times were not investigated), the absorbance of UV‐B and UV‐A radiation by CA and PE films, respectively, increased with time when wrapped around lamps. As a result, the irradiance of lamp/filter combinations decreased steadily (even when CA films were presolarized for 10 h), making it necessary to compensate by adjusting the height of the lamp bank or by changing filters frequently. Note that corrective action is required for UV‐A controls (PE films) as well as UV‐B experimental treatments (CA films). Changing filters is preferable, since aging of CA filters caused shifts in the ratio of UV‐B to UV‐A. However, in spite of these shifts, the normalized spectrum of weighted biologically effective UV‐B radiation did not change to a large extent.

Plasma appearance of labeled β-carotene, lutein, and retinol in humans after consumption of isotopically labeled kale

The bioavailability of carotenoids from kale was investigated by labeling nutrients in kale with 13C, feeding the kale to seven adult volunteers, and analyzing serial plasma samples for labeled lutein, β-carotene, and retinol. Ingested doses of labeled carotenoids were 34 μmol for β-carotene and 33 μmol for lutein. Peak plasma concentrations, areas under the plasma concentration-time curves (AUCs), and percentages of dose recovered at peak plasma concentrations were calculated. Average peak plasma concentrations were 0.38, 0.068, and 0.079 μM for [13C]lutein, [13C]β-carotene, and [13C]retinol, respectively. Average AUC values (over 28 days) were 42.8, 13.6, 13.2 μM h for [13C]lutein, [13C]β-carotene, and [13C]retinol, respectively. Percentages of dose recovered at peak plasma concentrations were 3.6, 0.7, and 0.7% for [13C]lutein, [13C]β-carotene, and [13C]retinol, respectively. A positive relationship was observed between baseline plasma retinol levels and [13C]retinol plasma response. It is possible that this relationship was mediated either through some aspect of β-carotene absorption or via the common pathways of metabolism for postdose and endogenous retinoid.

Comparison of the Response of Soybean to Supplemental UV-B Radiation Supplied by Either Square-Wave or Modulated Irradiation Systems

Soybean [Glycine max (L.) Merr.] cv CNS was grown in a field study at the Beltsville Agricultural Research Center, USDA, Beltsville, MD. Supplemental UV-B radiation was provided by two contrasting UV-B delivery systems, both of which were intended to simulate a 25% depletion of stratospheric ozone as estimated by an empirical model. In the first system, a seasonally-based supplemental UV-B irradiance was provided in a square-wave fashion (SQ) and in the second, a state-of-the-art modulated (MOD) UV-B supplementation system was employed.

Rapid Fluence-Dependent Responses to Ultraviolet-B Radiation in Cucumber Leaves: The Role of UV-Absorbing Pigments in Damage Protection

Summary The role of foliar UV-absorbing pigments (UVAP) as optical screening agents in the resistance of Cucumis sativus L. to UV-B radiation was investigated by exposing young leaves at a defined developmental stage from sensitive (cv Poinsett) and insensitive (cv Ashley) lines to brief UV-B treatments varying between 4 and 10 h. The amount of blue light (BL) or UV-A radiation during UV-B exposure was also varied. Rapid increases in UVAP immediately following UV-B were compared to damage in the same tissue (increased specific leaf weight or chlorosis) determined 72 h after the start of UV-B. Poinsett was more sensitive to both forms of UV-B damage than Ashley under conditions where the response to UV-B was not saturated. Although UVAP increased rapidly in response to UV-B, it is unlikely that optical screening by these compounds was responsible for genetic differences in sensitivity to UV-B for the following reasons: 1) the kinetics of UVAP increase were similar to that for induction of damage; 2) increases in UVAP in the UV-sensitive line (Poinsett) were similar to those in the resistant line (Ashley); and 3) BL and UV-A radiation significantly reduced damage by UV-B in cv Poinsett when given simultaneously but had relatively small stimulatory effects on rapid UVAP accumulation. These results do not rule out a general role for optical screening by UVAP nor do they exclude the possibility that qualitative differences in UVAP (e.g., as antioxidants) are the basis for cultivar differences.

Quantitative determination of 13C-labeled and endogenous β-carotene, lutein, and vitamin A in human plasma

Quantitative procedures employing liquid-chromatography/particle beam mass spectrometry (LC/PB-MS) and gas chromatography-mass spectrometry (GC-MS) were applied to the determination of the endogenous and 13C-labeled β-carotene, lutein, and retinol in plasma of a subject who consumed kale (Brassica oleracea) that had been grown in a 13CO2-enriched atmosphere. All compounds were analyzed in the negative chemical ionization (NCI) mode using methane as the moderating reagent gas. β-Carotene and lutein were analyzed using LC/PB-MS applying reversed-phase high-performance liquid chromatography (HPLC) separation procedures to resolve the analytes. The concentrations of the β-carotene isotopomers in the plasma over a several-week period were determined using 2H8-β-carotene as an internal standard. The total plasma concentrations of all trans-lutein were quantified by HPLC analysis with a photodiode array detector using β-apo-8′-carotenal as an internal standard, and the ratio of the 13C∶12C isotopomers of lutein was determined by PB-MS. The retinol isotopomers were collected from individual HPLC fractions of the plasma extract and then analyzed as the trimethylsilyl ethers by GC-MS in the NCI mode. The 13C-and 12C-retinol isotopomers were quantified using 2H4-retinol as an internal standard. These methods demonstrate the application of highly sensitive procedures empolying NCI MS for the quantitative determination of carotenoids and vitamin A for the purpose of conducting metabolism studies of phytonutrients.

PHOTOREGULATION OF ROOT: SHOOT RATIO IN SOYBEAN SEEDLINGS*

Abstract— The partitioning of plant growth between shoot and root has the potential to affect diverse physiological processes including water and nutrient uptake, nitrogen fixation, light interception, and interactions between plant and soil microorganisms. Root: shoot ratio is determined both by genetics and developmental status as well as by availability of water, nutrients and light. It is shown here that relative root growth was modulated by photomorphogenetic treatments designed to affect phytochrome (supplemental far‐red radiation given either as an end‐of‐day treatment or continuously during the photoperiod) or blue light photoreceptors (blue light‐deficient low pressure sodium lamps ± low irradiances of supplemental blue [i.e. 5% of total photon flux: 25 μ.mol m−2 s−1]). Photomorphogenetic control of root: shoot ratio was apparent within1–2 days when light treatments were initiated at emergence, and did not necessarily involve changes in net seedling growth. On the other hand, shortened daylength inhibited early seedling growth but had little effect on partitioning. Changes in relative root dry matter induced by supplemental far red radiation or blue light deficiency were similar to those caused by low irradiances, suggesting that phytochrome or blue light photoreceptors may be involved in regulating the partitioning of growth between shoot and root as a part of adaptation to vegetation shade. The influence of spectral quality on root: shoot ratio should be considered when comparing plants grown under different types of lighting or with different spacing.

Rhythms during extended dark periods determine rates of net photosynthesis and accumulation of starch and soluble sugars in subsequent light periods in leaves of Sorghum

Photosynthesis and photosynthate partitioning in leaves of Sorghum bicolor (L.) Moench exhibited a cyclic dependence on the duration (10–62 h) of dark periods inserted prior to bright light test periods (550 μmol·s-1·m-2, photosynthetic photon flux). Maximum rates of net photosynthesis and of accumulation of starch and soluble sugars were, in the order given, two-, three- and fourfold greater than minimum values. Between 14 and 53% of photosynthate was retained in leaves depending on the length of the dark period. These changes were sufficient to account for the previously described stimulatory effect of short daylengths (i.e., long nights) on carbohydrate accumulation in leaves (N.J. Chatterton and J.E. Silvius, 1980, Physiol. Plant. 49, 141–144). The freerunning periods for the rhythmic dependence on darkness, determined either directly or by curve fitting, were about 24 h for net photosynthesis, 23 h for starch accumulation, and 26 h for solublesugar cccumulation. The deviation from period lengths of 24 h for carbohydrate accumulation indicates that these rhythms are probably endogenous and circadian. Initial maxima were observed after 14 h of darkness for photosynthesis, after 18–22 h for starch, and after 26 h for soluble sugars. The differences in period length and phase indicate that at least three separate rhythms underlie the dependence of photosynthate partitioning in Sorghum on darkness. Periods of low leaf dry-matter accumulation coincided approximately with periods of high net photosynthesis. As a result, maximum photoassimination and maximum export were synchronized and, furthermore, occurred at about the same time as expected light periods.

Intrinsic Labeling of Plants for Bioavailability Studies

Fruits and vegetables are dietary components which convey numerous health benefits. Not only do fruits and vegetables contain many essential nutrients, but they also contribute to disease prevention. In the last decade, nutrition research has made great strides in identifying specific nutrients which have beneficial health effects and the mechanisms by which those health benefits are achieved. However, the ability of a specific nutrient to impart health benefits depends on the gastrointestinal tract’s ability to extract the nutrient from the plant material. The presence of a specific nutrient in a plant food is insufficient for providing health benefits if the bioavailability of that nutrient is very low. For example, spinach contains a high calcium content, but the presence of phytates and oxalates in spinach prevents the calcium from being absorbed in the gastrointestinal tract (Weaver et al., 1987; Heaney and Weaver, 1989; Peterson et al., 1992). Thus, calcium has a very low bioavailability from spinach.

PHOTOSYNTHATE PARTITIONING INTO Digitaria decumbens LEAF STARCH VARIES RHYTHMICALLY WITH RESPECT TO THE DURATION OF PRIOR INCUBATION IN CONTINUOUS DIM LIGHT

Abstract— In bright light the proportion of photosynthate stored as starch in leaves of Digitaria decumbens Stent, (pangolagrass) depends in a cyclic fashion on the duration of prior incubation in dim light. We suggest that a circadian rhythm, which runs during dim light conditions, controls the partitioning of photosynthate during subsequent bright light conditions. Varying the interval of dim light prior to transfer to bright light reveals this rhythm. The results emphasize the importance of endogenous timing mechanisms in relation to day and night length and the control of photoassimilate distribution and plant growth.